Call coverage (also sometimes referred to as call forwarding or call redirection from one endpoint to another) is a common feature of many telecommunications switches, particularly private branch exchanges (PBXs). An illustrative implementation of a call coverage feature is described in U.S. Pat. No. 4,436,962. Over time, efforts have been made to extend features across entire networks of routing nodes (e.g., switches) to make features that work with intra-node calls also work with inter-node calls. This is illustrated, for example, in U.S. Pat. No. 4,488,004. The call coverage feature has been a specific focus of some of these efforts, as illustrated by U.S. Pat. Nos. 5,313,459 and 5,369,695, for example.
As normally implemented on a single switch, the call coverage feature typically provides the covered endpoint with a coverage path which consists of a sequence of a plurality of alternative endpoints to which calls may be directed. Hence, if a call cannot be covered to (i.e., cannot be redirected and connected to) a first covering endpoint in the coverage path, the second covering endpoint is tried, and so on, until either coverage to one of the covering endpoints in the coverage path succeeds or all covering endpoints in the coverage path are exhausted. This has been a difficult capability to implement across a network of switches, however. A major reason for this difficulty is that, once a switch connects a call to another switch, it normally loses control of the call to the other switch. Therefore, if a first switch redirects a call for coverage to a covering switch but the covering endpoint served by the covering switch is unable to cover the call, the first switch is not able to regain control of the call and redirect it to the next covering endpoint specified by the applicable coverage path.
The first switch can try to obtain information from the covering switch regarding whether or not the covering endpoint served by the covering switch is available, prior to connecting the covered call thereto. However, this often requires that the switches of the network be interconnected by a separate control network through or from which the switches may obtain control and status information about each other and each other's endpoints. Moreover, even if the need for a separate control network can be avoided, it is virtually unavoidable that the obtained information may be outdated by the time the first switch takes action based thereon. Hence, the first switch may receive an indication from the covering switch that a remote coverage endpoint is available, but by the time it connects the covered call to the covering switch, the remote coverage endpoint may no longer be available. And, since the call has now been connected to the covering switch, the first switch has lost control thereof.
Therefore, what is needed is an arrangement that allows the call coverage feature to work in a network of switches the same way as it works in a single switch.